1. Field of the Invention
The present invention relates generally to inkjet printers and more specifically to a technique for controlling the application of fixer to a medium.
2. Background of the Invention
An inkjet printer forms a printed image by printing a pattern of individual dots on a printing medium. Inkjet printers print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles eject drops of ink at appropriate times pursuant to commands of a microcomputer or other controller.
Color thermal inkjet printers commonly employ a plurality of printheads, such as four, mounted in the carriage to produce different colors. Each printhead prints ink of a different primary color, with the commonly used colors being cyan, magenta, yellow, and black. Secondary or shaded colors are formed by depositing multiple drops of different primary color inks onto the same dot location (or nearby locations), with the overprinting of two or more primary colors producing secondary colors according to well established optical principles.
The printhead has an array of precisely formed nozzles attached to a printhead substrate that incorporates an array of firing chambers which receive liquid ink from the ink reservoir. In one type of printhead, each chamber has a thin-film resistor located opposite the nozzle so ink can collect between it and the nozzle. When electric printing pulses heat the resistor, a small portion of the ink vaporizes, causing a drop of ink to be ejected from the chamber. Proper sequencing of the firing resistors causes characters or images to be printed on the paper as the printhead moves across the paper.
Print quality is one of the most important considerations in the color inkjet printer field. Since the image output of an inkjet printer is formed of thousands of individual ink drops, the quality of the image is ultimately dependent upon the quality of each ink drop and the arrangement of the ink drops on the print medium.
One common problem that degrades the quality of the printed image is a lack of edge sharpness. In an ideal environment, ink drops would form a perfect circle of uniform size when applied to a medium. However, it is common for ink drops to bleed or feather into surrounding areas when applied to a medium. If the surrounding area is a non-ink area, then the resulting image will not have a well defined edge. If the surrounding area is another ink drop, then the colors of the two ink drops will combine, producing a different undesirable color. In either case, the quality of the image is seriously degraded.
Several methods have been employed to address this problem. The first method is to use special inks that will either react with each other or with the medium to improve edge sharpness. This method, however, severely restricts the types of inks that can be used in inkjet printing systems. The second method is to use special media. This method is also very restrictive since special media (e.g., specially purchased paper) must be used when printing.
A second common problem that degrades the quality of the printed image arises from slow drying of the ink. For example, after printing of a page is complete, the printer needs to hold onto the page for a predetermined time in order to let the ink dry before depositing the page in an output tray. This places an undesirable limit on how fast consecutive pages can be printed.
A third common problem that degrades the quality of the printed image is poor water fastness. After the ink has dried on its respective medium, it is desirable to maintain the integrity of the image even if a small amount of moisture, such as perspiration from one""s hand, is applied to the image. If the image has poor water fastness, the moisture will cause the ink to bleed or run, thereby seriously degrading the image.
These drawbacks have been addressed, in part, by using fixers. Fixers may be a clear solution or may even be dye-based ink printed beneath a pigment-based ink. Fixers allow inks to bond to a medium thereby improving edge sharpness. Fixers also help increase the drying speed of inks and improve water fastness.
Applying fixers to a medium can, however, cause undesirable effects. Applying too much fixer to each dot location can cause the medium to warp or cockle. Using too much fixer also increases the cost of printing a page since excess fixer is being used. On the other hand, using too little fixer can also cause undesirable effects. Too little fixer may not achieve the desired chroma, water fastness, strike through avoidance, and edge sharpness.
What is needed is an improved inkjet printer that applies an optimal amount of fixer in every dot location.
A system in an inkjet printer for determining the amount of a fixer to be applied to a medium is disclosed. The system receives image data, and a fixer plane generation circuit determines an amount of fixer to be applied to a dot location on a medium.